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DEC  26  I960 
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L161 — H41 


■ 

Reprinted  from  The  American  Breeders  Magazine , 
Vol.  HI , No.  4,  1912. 


Mad 

m, 


THE  FERTILITY  OF  HYBRIDS  IN  A MAMMA- 
T LIAN  SPECIES-CROSS 

John  Detlefsen 

(J>  Laboratory  of  Genetics,  Bussey  Institution,  Harvard  University 

Sterility  is  a common  phenomenon  in  the  hybrids  obtained  by 
mating  members  of  distantly  related  groups  or  types,  both  in  animals 
and  in  plants.  In  fact,  there  is  a tacit  understanding  among  the 
taxonomists  that  members  of  the  same  species  produce  fertile  off- 
spring when  mated  inter  se;  but  a successful  cross  between  members 
of  different  species  or  genera  may  result  in  sterility  of  one  or  both 
sexes  among  the  hybrids.  In  case  both  sexes  are  sterile,  a further 
genetic  study  becomes  impossible.  When  one  sex  alone  among  the 
hybrids  is  sterile,  that  sex  is  usually  the  male;  and  since  the  females 
are  fertile,  it  becomes  possible  to  study  the  inheritance  of  characters 
and  fertility  of  offspring  by  crossing  these  female  hybrids  back  to 
the  males  of  either  parent  species. 

Among  mammals,  at  least,  work  on  inheritance  and  fertility  in 
species  crosses  is  in  its  inception.  Various  compilers,  such  as  Rorig 
and  Przibram,  have  given  lists  of  mammalian  species  crosses,  with 
, J)rief  mention  of  the  partial  or  complete  dominance  of  one  parent, 
Oj  3-nd  the  fertility  of  the  hybrids,  when  known.  Grateful  as  we  are 
* c£  for  the  facts  that  are  thus  accumulated,  we  must,  nevertheless,  admit 
, 3 their  general  inadequacy;  for  most  mammalian  species  crosses  were 
m^de  by  those  who  were  merely  interested  in  sheer  possibility  of  the 
% ^ cross.  Those  breeders  who  are  interested  in  the  economic  mammals 
have  been  the  most  fortunate,  because  most  attention  has  been 
? directed  to  their  study.  The  consensus  of  opinion  is  that  the  time- 
rs! honored  cross  between  horse  and  ass  results  in  sterile  male  mules 
| but  that  the  female  mule  is  occasionally  fertile  with  either  the  horse 
or  ass  ( Waldo w von  Wahl,  1907).  The  zebroid  (zebra  X horse)  is 
supposedly  sterile  in  both  sexes  (Ewart,  1899;  Iwanoff,  1911).  The 
same  is  true  of  the  zebrule  (zebra  X ass).  When  the  cow  and  bison 
are  crossed,  they  produce  fertile  female  catteloes,  but  sterile  males 
(Boyd,  1908;  Iwanoff,  1911). 


■iftwinr 


American  Breeders  Magazine 


262 

f i V 1 1 H 

)The  offspring  of  the  fertile  female  mule  have  merely  been  men- 
tioned, but  further  reports  regarding  their  fertility  and  other  char- 
acters are  lacking.  The  female  hybrids  between  the  cow  and  bison 
have  been  crossed  back  to  males  of  both  parent  stocks,  thus  produc- 
ing one-quarter  bison,  or  three-quarters  bison.  The  one-quarter 
bison  females  are  fertile,  as  may  be  expected.  The  three-quarters 
bison  female  have  not  been  fully  tested,  but  are  possibly  also  fertile. 
The  one-quarter  bison  males  are  not  always  fertile,  for  Boyd  reports 
the  appearance  of  but  one  out  of  four  tested  males.  Iwanoff  reports 
a fertile  three-quarters  bison  male;  and  supposes,  on  purely  theoret- 
ical grounds,  that  a mating  of  such  a fertile  male  with  a quarter- 
bison  female  would  result  in  fertile  one-half  bison  of  both  sexes. 

Material. — It  has  been  the  good  fortune  of  the  writer  to  work  with 
the  progeny  of  a mammalian  species  cross  which  in  many  respects 
is  comparable  to  the  horse-ass  cross  and  bison-cow  cross.  It  is  my 
purpose  to  report  briefly  on  the  fertility  of  these  offspring.  The 
parent  species  were  the  wild  Brazilian  cavy  (Cavia  rufescens)  and 
the  domesticated  guinea-pig  (Cavia  porcellus).  The  two  forms  differ 
consistently  and  clearly  in  color,  texture  of  hair,  size,  shape  of  skulls 
and  skull  sutures,  tooth  formation,  and  the  like.  There  is  no  doubt 
but  that  the  two  parent  stocks  are  separate  species;  for,  even  if  the 
evidence  of  the  systematist  were  really  arbitrary,  such  evidence  must 
obtain  more  weight  when  one  considers  that  the  male  hybrids  are 
completely  sterile. 

The  original  crosses  between  these  two  species  were  the  result  of 
mating  the  wild  males  to  the  tame  females.  The  reciprocal  cross 
was  not  attempted,  for,  it  was  feared  that  the  smaller  wild  female 
would  succumb  in  pregnancy  when  mated  to  the  much  larger  sized 
tame  male.  The  wild  males  were  wholly  fertile,  with  tame  females, 
although  matings  were  secured  only  with  much  difficulty.  The 
tame  females  bore  their  hybrid  young  in  due  time  and  with  the  usual 
guinea-pig  average  per  litter.  Now  since  the  average  number  per 
litter  in  the  tame  guinea-pig  is  much  larger  than  in  the  wild,  and 
since  a tame  female  gives  this  larger  average,  even  when  impregnated 
by  a wild  male,  we  have  every  reason  to  believe  that  such  wild  males 
are  wholly  fertile  with  tame  females  and  the  abundant  number  of 
spermatozoa  insures  complete  fertilization.  Having  obtained  these 
one-half  wild  hybrids,  the  females  were  mated  back  to  the  wild  males 
and  the  tame  guinea-pig  males,  producing  three-quarters  wild,  and 
one-quarter  wild  respectively.  The  matings  to  the  wild  males  were 
not  very  successful  and  only  one  three-quarters  wild  male  was  reared 


Detlefsen:  Fertility  of  Hybrids 


263 


to  maturity.  He  was  sterile.  The  matings  to  the  tame  males  were 
wholly  successful,  and  produced  83  one-quarter  wild.  Pursuing  the 
same  method  of  mating  the  hybrid  females  of  one  generation  back 
to  the  tame  guinea-pig  males,  there  were  produced  a regular  series 
of  more  dilute  wild-blooded  generations  ranging  from  J wild  to  the 
3X2  wild.  In  all,  over  1700  hybrids  of  various  blood  dilutions  have 
thus  been  produced.  The  fertility  of  about  400  male  hybrids  has 
been  tested.  All  female  hybrids  are  fertile. 

Problems. — The  wild  males  were  wholly  fertile  in  captivity,  hence 
captivity  itself  may  be  eliminated  as  a factor  causing  the  sterility 
of  their  less  wild  hybrid  sons.  The  problem  immediately  suggested 
itself:  how  great  must  be  the  blood  dilution,  or  for  how  many  gen- 
erations must  the  hybrid  females  be  crossed  back  to  the  guinea-pig 
in  order  to  eventually  produce  fertile  male  hybrids?  When  fertile 
male  hybrids  were  produced,  would  all  their  offspring  be  fertile  in 
both  sexes,  if  such  males  were  mated  to  their  hybrid  sisters  or  guinea- 
pig  females? 

Method. — To  judge  an  animal’s  fertility,  the  breeding  test  is  hardly 
sufficient.  It  is  well  known  that  a male  may  be  potentially  fertile, 
and  yet  fail  to  show  it  because  of  some  physiological  state,  such  as 
the  emaciation  of  sickness  or  the  sluggishness  of  obesity.  Further- 
more the  number  of  males  to  be  tested  increased  so  rapidly  that 
facilities  were  lacking  to  breed  all  of  them.  Hence,  another  test 
was  devised.  By  making  a small  incision  in  the  scrotum  and  punc- 
turing the  epididymis  at  one  or  several  points,  and  placing  the  liquid 
contents  in  a normal  salt  solution  at  bodily  temperature,  with  the 
aid  of  a microscope  a complete  index  of  the  male’s  fertility  was 
obtained. 

Results. — Now,  whereas  any  male  always  gave  the  same  microscopic 
test  during  his  adult  life,  there  was  a great  difference  between  indi- 
vidual hybrids.  Some  males  might  not  possess  any  sperm  at  all; 
but  in  their  place  were  found  a few  or  many  incompletely  matured 
spermatogonia.  Other  males  might  possess  a fewnon-motile  or  motile 
spermatozoa  in  addition.  Still  others  might  have  an  abundance  of 
motile  spermatozoa,  just  as  any  normal  male.  All  grades  and  com- 
binations were  found;  but  the  last  class  alone  could  be  successfully 
mated  to  females.  Fully  200  offspring  from  such  males  have  been 
born. 

The  results  of  the  experiments  on  the  male  hybrids  up  through 
the  sixth  generation  are  given  in  the  table  presented.  The  one-half 
wild  hybrid  males  had  no  spermatozoa.  The  succeeding  generations 


264 


American  Breeders  Magazine 


of  less  intense  wild  males  present  a consistent  series,  in  which  a con- 
tinually increasing  percentage  of  males  show  spermatozoa.  Of  the 
21  males  tested  in  the  sixth  generation,  or  the  or  wild,  all  had  sper- 
matozoa. But  the  mere  presence  of  sperm  does  not  produce  fertile 
males.  In  order  that  fertilization  of  the  egg  shall  take  place,  the 
sperm  must  be  motile  to  reach  and  penetrate  the  egg.  Many  males 
with  immotile  sperm  were  mated  to  females,  but  invariably  gave  the 
same  result:  no  progeny. 

When  we  consider  those  males  which  had  any  motile  spermatozoa 
whatsoever,  we  find  the  same  sort  of  a series.  The  ^ wild  hybrid 
males  had  no  sperm  and  naturally  would  have  none  which  were 
motile.  The  \ wild  males  likewise  had  no  motile  sperm,  although  we 
saw  in  the  previous  column  that  25  per  cent  showed  sperm.  The 
J wild  males  were  the  first  which  showed  motile  sperm,  and  were 
likewise  the  first  to  be  successfully  mated  with  females.  The  per- 


Table  of  fertility  of  hybrid  males. 


Class  of  hybrids. 

Total  number 
tested. 

per  cent  with 
any  sperm. 

per  cent  with 
any  motile 
sperm. 

per  cent  readily 
fertile. 

i wild 

6 

0 

0 

0 

J wild 

22 

25.0 

0 

0 

i wild 

71 

47.8 

17.3 

9.8 

A wild 

94 

71.1 

46.6 

35.5 

tjt  wild 

89 

88.7 

62.9 

60.7 

& wild 

21 

100.0 

66.7 

66.7 

centage  of  males  with  motile  sperm  increased  rapidly  in  each  succeed- 
ing generation  until  finally  the  -g-1*  wild  showed  66.7  per  cent  with 
motile  sperm.  So  far  as  I have  been  able  to  test,  it  would  seem  that 
any  male  with  motile  spermatozoa  is  fertile;  but  in  those  cases  in 
which  immatured  spermatogonia  or  non-motile  spermatozoa  greatly 
outnumber  the  motile  spermatozoa,  the  chances  that  such  will  reach 
and  penetrate  an  egg  are  small.  An  intimate  study  of  the  motility 
of  sperm  and  the  possibilities  of  obtaining  offspring  from  male  hybrids, 
has  led  me  to  believe  that  any  male  with  an  abundance  of  motile  sperm 
is  readily  fertile.  Abundance  of  motile  sperm  means  at  least  one- 
half  motile.  The  last  column  gives  the  percentages  of  male  hybrids 
in  each  generation,  which  are  readily  fertile  and  which  can  successfully 
impregnate  females.  This  last  category  shows  the  same  sort  of 
increase  that  the  others  show.  It  is  therefore  clear  that  fertile  male 
hybrids  may  be  produced  in  constantly  increasing  numbers  in  the 
offspring  of  a cross  which  originally  gave  only  sterile  males  and  fertile 


Laughlin:  Eugenics  Field  Workers  Conference  265 

females.  In  the  original  cross,  elements  are  introduced  or  formed 
which  prevent  the  full  maturation  of  the  male  reproductive  cells,  but 
the  female  reproductive  cells  seem  unaffected.  These  disturbing 
elements  may  be  eliminated  by  continually  crossing  the  female  hybrids 
back  to  normal  tame  males,  thus  producing  fertile  male  hybrids.. 

Offspring  of  fertile  male  hybrids. — It  is  not  out  of  place  to  mention 
the  fertility  of  the  sons  of  the  fertile  male  hybrids.  The  male  hybrids 
with  an  abundance  of  motile  spermatozoa  could  be  successfully 
mated  to  female  hybrids,  and  to  tame  female  guinea-pigs. 

When  a fertile  male  hybrid  was  mated  to  a female  hybrid  their 
male  offspring  were  not  necessarily  fertile.  We  hardly  expected 
they  would  be,  for  the  female  might  transmit  the  disturbing  elements 
in  this  cross  just  as  much  as  when  mated  to  a tame  guinea-pig.  About 
forty  male  hybrids  from  this  sort  of  a cross  have  been  tested,  and 
they  give  all  grades  between  absolute  sterility  and  fertility. 

When,  on  the  other  hand,  a fertile  male  hybrid  was  mated  to  the 
guinea-pig  female,  all  the  male  offspring  have  been  fertile.  This  is 
the  expected  outcome,  for  the  fertile  male  hybrid  may  be  regarded 
as  a sort  of  recessive,  in  which  the  disturbing  elements  introduced 
in  the  original  cross  have  been  eliminated;  and  when  he  is  mated 
to  the  female  guinea-pig,  no  such  elements  are  again  introduced. 
About  thirty  male  hybrids  from  this  class  of  crosses  have  been  tested 

and  all  found  to  be  wholly  fertile. 

Practical  application. — If  the  cattaloes,  mules,  and  other  mamma- 
lian hybrids  are  at  all  comparable  to  the  hybrids  in  these  experiments, 
then  fertile  races  of  such  hybrids  may  be  produced  in  the  same  man- 
ner. As  a simple  illustration,  I may  say  that  all  the  color,  coat,  size, 
and  anatomical  characters  known  in  guinea-pigs,  have  been  trans- 
ferred to  these  hybrids.  Any  combination  of  these  characters  may 
be  united  with  fertility.  It  is  conceivable  that  desired  characters 
in  hybrids  between  other  mammalian  species  may  be  combined  with 
fertility  of  both  sexes,  in  the  same  manner. 


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